The subject invention relates to the extrusion of molten thermoplastic film onto a quench roll casting drum.
In the production of film from certain thermoplastic polymers in which such film is formed at temperatures above or near the polymer melting point, it is important to quickly cool the freshly formed film to a temperature below the second order transition temperature, i.e., that temperature at which, as temperature decreases, the polymer goes from a flexible state to a more rigid, glassy state, in order to prevent crystallization which would interfere with subsequent orientation of the film. It is, therefore, important to achieve quick, intimate contact between the extruded film and the quench roll, hence preventing excess air from being trapped between the film and the roll. A number of devices have been employed to create more initmate contact between the extruded film and the quench roll, the most prominent being, generally, forced air pinning, electrostatic pinning, and vacuum pinning. Forced air pinning, where the molten film is urged against the quench roll by means of air pressure exerted against the upper, or leading surface of the extruded film, is of limited use during high speed operation due to inherent limitations in the magnitude of air pressure which can be applied to a film with low melt tensions. Electrostatic pinning, where the extruded film is held to the quench roll by means of electrostatic forces, has also been limited to slower operation as air bubbles are trapped under the film at high speeds.
In the recent past, the removal of air proximate the line of contact between the extruded film and the quench roll has been attempted by use of various vacuum box devices, for example, the vacuum devices disclosed in U.S. Pat. No. 3,347,962 (Dieck et al) and U.S. Pat. No. 4,310,295 (Heyer), the latter hereby incorporated by reference. A major limitation of such vacuum devices, however, is the narrow operating range of vacuum levels that can be exerted on the lower, or trailing surface of the molten film for any given casting speed, particularly in the manufacture of thin gauge films. As detailed below, a variation in vacuum levels or variation in casting speed outside of this delicate operating range results in dramatic irregularities in cast film quality.